1. Field of the Invention
The present invention relates to a high-capacity magnetic recording medium, and particularly relates to a magnetic recording medium having superior recording/reproducing characteristics and transport characteristics.
2. Description of the Related Art
Magnetic tapes, which are a type of magnetic recording media, have various applications such as audiotape, videotape, and computer tapes. In particular, in the field of computer data backup tapes, with the increase in the capacity of hard disks to be backed up, tapes having a recording capacity as large as several hundred GB per reel have been put on the market. In the future, in order to support further increases in the capacity of hard disks, it is essential to increase the capacity of such data backup tapes.
With regard to a magnetic tape for use as a data backup tape, the recording wavelength has been shortened with the further increase in the recording capacity, and in order to suppress deterioration in the recording/reproducing characteristics depending on the spacing between a magnetic head and the magnetic tape, the surface of the magnetic layer has become increasingly smoothed. The smoothed surface of the magnetic tape results in an increase in the contact area between the magnetic head and the magnetic tape, and thus the friction between the head and the tape increases.
Moreover, in such a magnetic tape whose magnetic layer has a smoothed surface, repeated transport caused the surface of the magnetic layer to be further smoothed due to sliding against the magnetic head. Thus, the coefficient of friction increased or sticking tended to occur, resulting in unstable transport, or in extreme cases, the magnetic tape might be cut, or the magnetic head might be damaged.
To address such issues, for example, JP 2006-127666A proposes a method of avoiding sticking of a magnetic tape to a magnetic head by forming minute recesses in a BOT part (BOT: beginning of tape) and an EOT part (EOT: end of tape) of the magnetic tape that are particularly likely to stick to the magnetic head. Moreover, JP 2008-262645A proposes a method of avoiding sticking of a magnetic tape to a magnetic head by vibrating the magnetic head when starting or stopping transport during recording/reproduction of the magnetic tape.
However, both of the technologies disclosed in JP 2006-127666A and JP 2008-262645A propose the avoidance of sticking of the magnetic tape at the specific positions of the magnetic tape or when the magnetic tape is in the specific states, and do not propose a countermeasure to an increase in the coefficient of friction or sticking at other positions or in other states.
As a magnetic recording medium that solves the problems as described above, for example, JP 9-69224A proposes a magnetic recording medium in which a magnetic layer has a surface roughness of 5 nm or less, a lubricant portion accounts for 20 to 60% of the surface of the magnetic layer, and the lubricant portion has an average thickness of 1 to 10 nm. Moreover, JP 2003-132516A proposes a magnetic recording medium in which a magnetic layer has a surface lubricant index within a range of 1.3 to 5.0 and a center surface average roughness (i.e., surface roughness) of a 40 μm×40 μm area as measured by an atomic force microscope (AFM) is 4 nm or less.
The magnetic recording medium proposed in JP 9-69224A is a single-layer magnetic disk in which a magnetic layer having a thickness of 1 μm is formed on a substrate, and it is disclosed that the surface roughness of the magnetic layer can be set to 5 nm or less in order to achieve sufficient durability. However, in all the embodiments disclosed in JP 9-69224A, the magnetic layer has a surface roughness of 2 nm or more, and it is demanded to reduce the surface roughness further in order to improve the recording/reproducing characteristics even more.
Moreover, with regard to the magnetic recording medium proposed in JP 2003-132516A, even though the surface lubricant index determined by Auger electron spectroscopy corresponds to the amount of lubricant in the surface, it is not the thickness itself of a lubricant layer. Moreover, JP 2003-132516A discloses that in order to suppress the occurrence of noises, the surface roughness of the magnetic layer should be set to 4 nm or less and more preferably 3 nm or less. However, in all the embodiments disclosed in JP 2003-132516A, the magnetic layer has a surface roughness of 2.1 nm or more, and it is demanded to reduce the surface roughness further in order to improve the recording/reproducing characteristics even more.